1. Field of the Invention
The present invention relates to annulus pressure responsive downhole tester valves. Particularly, the present invention provides a mechanical means for locking the tester valve in a chosen position during subsequent changes in well annulus pressure.
2. Description of the Related Art
The related art includes a variety of downhole tools such as testing valves, circulating valves and samplers that are operated in response to a change in well annulus pressure. One particular type of annulus pressure responsive tool has previously been developed by the assignee of the present invention and is generally referred to as a low pressure responsive tool.
An example of such a low pressure responsive tester valve is shown in U.S. Pat. No. 4,667,743 to Ringgenberg et al. The low pressure responsive tool includes a ball-type tester valve operatively associated with a power piston having first and second sides communicated with the well annulus through first and second pressure conducting passages defined in the tester valve. A retarding means, such as a metering orifice, is placed in the second pressure conducting passage for delaying communication of a change in well annulus pressure to the second side of the power piston for a sufficient time to allow a pressure differential at the first side of the power piston to move the power piston downward. After a period of time, a pressure differential is built up at the second side of the power piston to move it upward. The movement of the power piston is typically accommodated by compression of a compressible gas such as nitrogen.
It is desirable with such tools to be able to selectively lock the power piston and the associated operating element of the tool in a chosen position so as to disable them during subsequent changes in well annulus pressure.
A hydraulic means for locking the tool is shown in U.S. Pat. No. 5,180,007. During normal operation of this type of tool, well annulus pressure is cycled between hydrostatic pressure and an increased first level above hydrostatic pressure to move a power piston and tester valve between the closed and open positions of the tester valve. The tester valve may be retained in an open position during reduction of well annulus pressure back to hydrostatic pressure by opening a bypass past the power piston, thereby deactivating the power piston. While the bypass is open, well annulus pressure can be decreased without moving the tester back to its closed position. The bypass is opened in response to increasing well annulus pressure to a second level which is higher than the first level. The power piston may be reactivated when the well annulus pressure is again raised to the second level. Hydraulic locking systems are advantageous in that they permit a tool to be held in a chosen position for an infinite number of well annulus pressure cycles. Current hydraulic locking designs, however, may be less reliable and more difficult to manufacture. Component parts for the bypass means are small and may be difficult to manufacture to precise dimensions at a low cost. Also, the complexity of the flow paths of the bypass means may provide a reliability problem. Metering through this bypass means may cause great variability in the upward travel of the actuating piston. The piston may fail to fully return to its initial position, causing premature activation of the "lock open" feature.
Mechanical position control schemes are known which use devices such as a lug and slot ratchet assembly attached to the power piston like that shown in Ringgenberg et al. U.S. Pat. No. 4,667,743. One disadvantage of this type of arrangement is that the power piston must move through a predetermined series of movements in order to obtain a selected position, as is determined by the various positions defined on the ratchet assembly. Also, the tool is only held in a chosen position for a predetermined number of well annulus pressure cycles. In addition, the pressure forces which open and close the valve both act across the power piston. As a result, subjecting the tool to pressure differentials across the power piston which are too great may damage the lugs of the ratchet assembly during opening or closing of the ball valve. The tool may become unreliable, difficult to operate or inoperable.
In another aspect, metering valve assemblies known in the art are inherently limited to the relatively narrow range of pressure differentials the valve assemblies are manufactured to be operable in response to. For example, a metering valve assembly which is designed to operate at a 5,000 psi pressure differential will be operable only around that range. If it is desired to operate a tester valve in well conditions at which a 10,000 psi differential exists, the tool must be disassembled to replace the metering valve assembly with one operable at a higher pressure differential. The oil and nitrogen contained within the tool is lost, and these fluids must be replaced.